Ring-bowl (RB) mills are used in coal-fired furnaces because pulverized coal burns substantially like gas and, therefore, fires are easily lighted and controlled. Pulverized coal furnaces can be readily adapted to burn all coal ranks from anthracite to lignite.
When in operation, raw coal enters the pulverizer through a center feed pipe onto a rotating bowl. Centrifugal force causes the coal to move outward from the center and under journal assemblies, where the coal is crushed by large rolls. The partially pulverized material passes over the rim of the bowl, where the coal is entrained by a rising hot-air stream and pyrites and tramp iron that enter the mill with the coal fall into the millside. The rejected materials are swept out of the mill into an external hopper. The air-transported partially pulverized coal is classified on the basis of size, with the larger, heavier particles being returned to the bowl and coal having the desired particle size exiting the pulverizer.
Referring to FIG. 1, the rotating bowl is gear driven by a gear unit (e.g., gearbox) 2 disposed below and external to a mill housing 3. Since the gear unit 2 does not penetrate the mill housing 3, it is not directly exposed to the pulverized coal entrained in the primary air. A mill-housing penetration seal 5, above the gear unit 2 prevents airborne coal particles from settling into the gear unit 2. Conventionally, the mill-housing penetration seal includes a mill base seal ring 5 disposed between the movable grinding-bowl support hub (bowl hub) 4 and a stationary mill base hub 6. In addition, a mill base hub bushing 7 and thrust bearing 8 are disposed between the movable bowl hub 4 and a stationary mill base hub 6. This mill base hub bushing 7 is typically a bronze bushing.
With a switch to Powder River Basin (PRB) coal, e.g., from Powder River Basin, Wyoming, many older RB suction mills are reaching their thermal limits when trying to make rated capacity and are running close to positive pressure under the bowl. Taking a derate is not an option at many plants. To assist in drying the coal during wet and cold times of the year, many plants add duct burners to the system. However, this can add a significant heat load to the mill. The bowl hub 4 and mill base hub 6 conducts this additional heat load, especially if the millside 9 is not lined, as is the case with many of these older units.
A positive pressure underbowl condition forces coal past the existing mill base seal ring 5 and causes excessive coal contamination of the upper thrust bearing 8 and gearbox lubricant. These conditions can lead to premature failure of the upper thrust bearing 8 and coal contamination of the entire gearbox 2 and all of the gears and bearings in the gearbox.
The additional heat provided by the duct burners can also cause rapid oxidation of the lubricant in the gearbox, especially near the upper thrust bearing 8. This oil degradation can lead to rapid failure of the upper thrust bearing 8 and damage to the other bearings and gears in the gearbox.
In extreme cases, the thermal gradients caused by firing the duct burners is large enough to cause the bronze mill base hub bushing OD to expand faster than, and interfere with, the stationary mill base hub ID. The faster expansion of the mill base hub bushing OD causes a lock-up and failure of the entire gearbox unit.
Accordingly, there is a desire to improve the bearing/sealing interface between the mill base hub 6 and bowl hub 4 to prevent a lock-up and failure of the entire gearbox unit caused by additional heat load, as well as to further prevent coal contamination in the gearbox unit.